How much water does the vase hold? I would have to say, unless it is very wide, that it's probably not a good idea. Having a large amount of water with very little surface area in contact with the air means poor gas exchange. It also limits the amount of space the betta has for air access in order to breathe (bettas in low-oxygen environments need to be able to breathe air from the surface of the water). Also, it would be difficult to fit a heater and filter into a vase, unless it is very large.